We are offering Series A units (the “Series A Units”), with each Series A Unit consisting of (i) one share of common stock and (ii) one Series J Warrant to purchase of a share of common stock (the “Series J Warrants”).

The Series A Units will not be issued or certificated. The shares of common stock and Series J Warrants will all be issued separately and are immediately separable. This prospectus supplement also relates to the offering of the shares of common stock issuable upon exercise of Series J Warrants.

Each Series J Warrant will be exercisable at any time on or after the issuance date until the five-year anniversary of the issuance date. Each Series J Warrant will be exercisable at a price of $ per share of our common stock, subject to adjustment.

For a more detailed description of our common stock and Series J Warrants, see the section entitled “Description of the Securities We are Offering” beginning on page S-10 of this prospectus supplement.

Our common stock is quoted on the NYSE MKT under the symbol “PTN.” On November 29, 2016, the closing price of the common stock was $0.82. We do not intend to list the Series J Warrants to be sold in this offering on any securities exchange.